In recent years, with an increase in functions of semiconductor laser devices, integrated semiconductor laser devices in which a plurality of semiconductor laser chips are stacked to produce high-power output light have been developed.
FIG. 10 is a front view illustrating a conventional integrated semiconductor laser device 1000 including a heat sink 6. A first semiconductor laser chip 1 is bonded to the heat sink 6 with solder 7. Likewise, a second semiconductor laser chip 2 is bonded to the first chip 1, a third semiconductor laser chip 3 is bonded to the second chip 2, and a fourth semiconductor laser chip 4 is bonded to the third chip 3, with the solder 7. Reference numerals 5a to 5d designate light emitting regions, i.e., active layers, of the semiconductor laser chips 1 to 4, respectively. Reference numerals 1a, 2a, 3a, and 4a designate light emitting facets of the semiconductor laser chips 1, 2, 3, and 4, respectively. Laser light beams are emitted from the light emitting regions 5a to 5d in the direction perpendicular to the light emitting facets 1a to 4a of the laser chips 1 to 4, respectively, i.e., in the direction perpendicular to the paper on which FIG. 10 appears. An upper electrode (not shown) is disposed on the fourth semiconductor laser chip 4 and connected to a power supply (not shown) with wires (not shown), such as Au wires. In addition, the heat sink 6 is connected to a power supply (not shown) with wiring (not shown).
The integrated semiconductor laser device 1000 is generally employed as a light source for a laser radar, and it outputs laser light at the watt level in a pulsed operation. Usually, when a semiconductor laser chip is continuously operated, the laser chip is thermally saturated at a certain level of output power and, thereafter, the output power decreases. In the integrated semiconductor laser device 1000, since four semiconductor laser chips 1 to 4 are simultaneously operated in a pulsed operation, high-power laser light at the watt level is realized as the total laser light emitted from these laser chips 1 to 4. The integrated semiconductor laser device 1000 is provided with a condensing lens (not shown) for collecting the laser beams emitted from the respective semiconductor laser chips 1 to 4. The condensing lens is located in front of the light emitting facets of these laser chips.
The semiconductor laser chips 1 to 4 are stacked one on another through the solder 7 to make the integrated semiconductor laser device 1000 because a semiconductor laser chip is usually longer in the horizontal direction than in the vertical direction, i.e., the width is larger than the height. Therefore, the distance between adjacent light emitting regions of the laser chips is shorter in the laser chips stacked in the vertical direction than in laser chips arranged in a horizontal direction. As a result, the broadening of the light applied to the condensing lens is reduced.
However, the prior art integrated semiconductor laser device 1000 and the method making it still have problems to be solved. FIGS. 11(a) and 11(b) are a plan view and a front view, respectively, for explaining these problems. In fabricating the integrated semiconductor laser device 1000, when the semiconductor laser chips 1 to 4 are successively stacked so that an upper electrode of a lower laser chip is soldered to a lower electrode of a higher laser chip, if a force is applied to the semiconductor laser chips in the horizontal or diagonal direction while the solder 7 is molten, the directions of the light emitting facets 1a to 4a of the laser chips 1 to 4 unfavorably vary from each other in the completed laser device 1000 as shown in FIGS. 11(a) and 11(b). Therefore, laser beams are emitted from light emitting regions 5a to 5d in different directions and the power of the laser light collected by the condensing lens is reduced.
Further, as illustrated in FIG. 11(b), during soldering of the laser chips, the molten solder 7 may flow over the side surface of the chip and contact the active layer exposed at the side surface. In this case, when the semiconductor laser device 1000 is operated, the active layer is electrically short-circuited by the solder 7, and desired laser operation is not achieved.
Since the semiconductor laser chip 1 contacts the heat sink 6, unwanted heat generated in the semiconductor laser chip 1 during operation is conducted through the heat sink 6, whereby the semiconductor laser chip 1 is cooled. However, heat generated in the laser chips above the laser chip 1 is not conducted through the heat sink 6, resulting in an excessive rise in temperature that adversely affects the performance of the laser device 1000. As a result, the lasers device 1000 cannot emit high-power laser light with high stability for many hours.
Since the semiconductor laser chip employed in the integrated semiconductor laser device 1000 is about 300 square microns in area at the front (rear) surface, successive stacking and soldering of the semiconductor laser chips are very complicated. In addition, it is difficult to automate the process. On the other hand, an apparatus for fabricating an ordinary semiconductor laser device in which a semiconductor laser chip is soldered to a heat sink plate, i.e., a mount, has already been developed and put to practical use.